The present invention relates to a predistorter that compensates for distortions generated by a power amplifier for power amplifying high-frequency signals and, more particularly, to a multi-band predistorter using power series representation (hereinafter, multi-band power series type predistorter) that can be used in common to multiple frequency bands to implement distortion compensation of high-frequency signals of the respective frequency bands.
One of methods that have been proposed to compensate for nonlinear distortions generated by microwave power amplifiers is a predistortion method using a digital signal processing scheme (hereinafter referred to as a digital predistortion method) (for example, UK Patent Application, GB2,335,812A, hereinafter referred to as patent document 1). The digital predistortion method features elimination of the need for complex analog circuitry by implementing a predistorter configuration through utilization of digital signal processing.
There has been known a digital predistorter of the type having a look-up table for pre-linearizing amplifier nonlinearities (for example, H. Girard, and K. Feher, “A New Baseband Linearizer for More Efficient Utilization of Earth Station Amplifiers Used for QPSK Transmission,” IEEE J. Select., Area Commun., SAC-1, No. 1, 1983, hereinafter referred to as non-patent document 1, and Japanese patent application laid open No. 2002-522989, hereinafter referred to as patent document 2). The digital predistorter with the look-up table updates values set in the look-up table by feeding back thereto the amplifier output signal to keep the distortion component down under a set value. It is well-known in the art that distortion compensation can be achieved by such digital signal processing, and that the amount of compensation is equal to or smaller than approximately 15 dB (for example, Ishikawa, Hase, Kubo, Tozawa, and Hamano, “Development of Adaptive Predistortion Amplifier for W-CDMA Base Station,” 2002 Assembly of the Society of Electronics, Information and Communication Engineers of Japan, C-2-31, 2002, 09, hereinafter referred to as non-patent document 2).
There has also been proposed a predistorter based on a power series model. This predistorter has been implemented by use of analog circuitry and has yielded distortion-suppression performances in excess of 30 dB (non-patent document 3). It is known that the power series model successfully models amplifier nonlinearities with high accuracy (for example, Tri T. Ha, “Solid-State Microwave Amplifier Design,” Chapter 6, Krieger Publishing Company, 1991, hereinafter referred to as non-patent document 4). With the distortion compensating method for the digital predistorter using the power series model, it is necessary to extract signals for correcting coefficients of respective order from the amplifier output signal. In patent document 1 the fundamental wave and distortions of respective orders are eliminated or cancelled from the transmission signal to extract such correcting signals. A method for extracting the correcting signals of the power series model with more ease is to use two carrier waves of the same level as pilot signals (for example, T. Nojima, and T. Kanno, “Cuber Predistortion Linearizer for Relay Equipment in 800 MHz Band Land Mobile Telephone System,” IEEE Trans. Vech. Tech., Vol. VT-34, No. 4, pp. 169–177, 1985, 11, hereinafter referred to as non-patent document 3).
In a mobile radio communication system that simultaneously performs communication services in two or more discretely separated frequency bands, it is necessary to use predistorters each adapted to meet conditions in a particular frequency band. However, the frequency range over which the conventional predistorter configuration can be adjusted is only about 20 MHz or so about the carrier frequency, and hence, in the PDC (Personal Digital Cellular) system that transmits signals of 800 MHz and 1.5 GHz bands, it is impossible to provide predetermined compensation of the both transmission signals. The conventional predistorter is not provided with a distortion generation path for making distortion compensation satisfactory to every transmission frequency band, and for this reason, it cannot make adjustment for implementing sufficient compensation in the multiple frequency bands. Predistortion processing for multiple frequency bands by one predistorter will ensure realization of a simplified, power consumption saving and miniaturized equipment configuration.
There is also known a method in which multiple power series type predistorters, each having distortion generating means corresponding to one of multiple frequency bands, are configured with a delay line of the prior-art power series type predistorter used in common to the predistorters. With such a method in which the multiple power series type predistorters are configured as if to be in parallel relation to the delay line, transmission signals of the multiple frequency bands are input to the respective distortion generating means. The respective distortion generating means each adjust the amplitude and phase of the input transmission signal in a manner to implement distortion compensation for each particular frequency band. When transmission signals of multiple frequency bands are input to the distortion generating means, it is impossible to make optimum adjustments to the amplitude and phase of the input transmission signal of every frequency band. For example, when the transmission signals are of 800 MHz and 1.5 GHz bands, the distortion generating means is capable of setting optimum amplitude and phase for the transmission signal of the 800 MHz band, but for setting optimum amplitude and phase for the signal of the 1.5 GHz band which is spaced 700 MHz apart in frequency from the 800 MHz, it is necessary to use amplitude and phase setting means capable of operating at a speed high enough to overcome the 700-MHz difference of frequency. But such high-speed amplitude and phase setting means is not available. Thus, even if multiple power series type predistorters each operable in a particular frequency band are used, it is impossible to implement a power series type predistorter configuration operable in multiple frequency bands.